Lubricants in commercial use today are prepared from a variety of natural and synthetic base stocks admixed with various additive packages and solvents depending upon their intended application. The base stocks can include, e.g., Groups I, II and III mineral oils, gas-to-liquid base oils (GTL), Group IV polyalpha-olefins (PAO) including but not limited to PAOs made by using metallocene catalysts (mPAOs), Group V alkylated aromatics (AA) which include but are not limited to alkylated naphthalenes (ANs), silicone oils, phosphate esters, diesters, polyol esters, and the like.
Manufacturers and users of lubricating oil compositions desire to improve performance by extending oil drain life of the lubricating oil composition. Extended drain life is a highly desirable marketing feature of lubricating oil compositions, especially those containing Group IV/Group V base stocks.
Shear stability of the lubricating oil composition affects the oil drain life of the lubricating oil composition, especially those experiencing high-shear stress events during normal use such as gear box oils. Oxidative degradation of lubricating oil composition can lead to damage of metal machinery in which the lubricating oil composition is used. Such degradation may result in deposits on metal surfaces, the presence of sludge, or a viscosity decrease or change in the lubricating oil composition. For gear box oils, significant loss of viscosity during life of the oil can lead to reduced efficacy in lubrication, and hence premature wear and failure of the gears.
The kinematic viscosity of a lubricating oil composition is partly related to the antioxidation performance and degree of oxidation of the lubricating oil composition. A lubricating oil composition being used in machinery has experienced oxidative degradation when the kinematic viscosity of lubricating oil composition reaches a certain level, and the lubricating oil composition needs to be replaced at that level. Improving the oxidation stability and antioxidation performance of the lubricating oil composition improves the oil drain life by increasing the amount of time the lubricating oil composition can be used before being replaced. Various approaches are used to improve the antioxidation performance and extend the oil drain life of Group IV/Group V lubricating oil compositions. The approaches typically involve increasing the antioxidant additive concentrations of the lubricating oil composition.
US 2013/210996 discloses a PAO having a kinematic viscosity at 100° C. of 135 cSt or greater that is derived from not more than 10 mol % ethylene and characterized by a high shear stability demonstrated by, after being subjected to twenty hours of a taper roller bearing testing, having a kinematic viscosity loss of less than 9%. In certain preferred examples in this patent reference, the PAO comprises no more than 5.0 wt % of the polymer having molecular weight of greater than 45,000. It is disclosed that a low concentration of large PAO molecules (e.g., those having molecular weight of at least 45,000) in the PAO base stock is desired for a high shear stability characterized by a low kinematic viscosity loss after severe shear stability tests.
The above reference is primarily concerned with the shear stability of a single base stock material put into a lubricant oil composition. However, it has been found that, surprisingly, when multiple base stocks or other oil components are mixed, even if each of them exhibits exceedingly low shear loss when tested individually in prolonged shear stability test under severe test conditions, the mixtures of them may exhibit appreciable shear loss when tested under similar conditions. This shows that the various components may interact with each other in the oil, forming shear-unstable objects.
Therefore, there remains the need for oil compositions comprising multiple oil components that exhibit, among other desired properties, a high shear stability. The present invention satisfies this and other needs.